Fix surround bug when cursor on same pair

For example when the cursor is _on_ the `'` in `'word'`, the cursor
wouldn't move because the search for a matching pair started _from_ the
position of the cursor and simply found itself.

helix-core/src/surround.rs

@@ -41,11 +41,14 @@ pub fn find_nth_pairs_pos(
     let (open, close) = get_pair(ch);
 
     let (open_pos, close_pos) = if open == close {
-        // find_nth* do not consider current character; +1/-1 to include them
-        (
-            search::find_nth_prev(text, open, pos + 1, n, true)?,
-            search::find_nth_next(text, close, pos - 1, n, true)?,
-        )
+        let prev = search::find_nth_prev(text, open, pos, n, true);
+        let next = search::find_nth_next(text, close, pos, n, true);
+        if text.char(pos) == open {
+            // curosr is *on* a pair
+            next.map(|n| (pos, n)).or_else(|| prev.map(|p| (p, pos)))?
+        } else {
+            (prev?, next?)
+        }
     } else {
         (
             find_nth_open_pair(text, open, close, pos, n)?,
@@ -198,6 +201,11 @@ mod test {
         assert_eq!(find_nth_pairs_pos(slice, '\'', 13, 1), Some((10, 15)));
         assert_eq!(find_nth_pairs_pos(slice, '\'', 13, 2), Some((4, 21)));
         assert_eq!(find_nth_pairs_pos(slice, '\'', 13, 3), Some((0, 27)));
+        // cursor on the quotes
+        assert_eq!(find_nth_pairs_pos(slice, '\'', 10, 1), Some((10, 15)));
+        // this is the best we can do since opening and closing pairs are same
+        assert_eq!(find_nth_pairs_pos(slice, '\'', 0, 1), Some((0, 4)));
+        assert_eq!(find_nth_pairs_pos(slice, '\'', 27, 1), Some((21, 27)));
     }
 
     #[test]

helix-core/src/textobject.rs

@@ -247,14 +247,13 @@ mod test {
                 "samexx 'single' surround pairs",
                 vec![
                     (3, Inside, (3, 3), '\'', 1),
-                    // FIXME: surround doesn't work when *on* same chars pair
-                    // (7, Inner, (8, 13), '\'', 1),
+                    (7, Inside, (8, 13), '\'', 1),
                     (10, Inside, (8, 13), '\'', 1),
-                    // (14, Inner, (8, 13), '\'', 1),
+                    (14, Inside, (8, 13), '\'', 1),
                     (3, Around, (3, 3), '\'', 1),
-                    // (7, Around, (7, 14), '\'', 1),
+                    (7, Around, (7, 14), '\'', 1),
                     (10, Around, (7, 14), '\'', 1),
-                    // (14, Around, (7, 14), '\'', 1),
+                    (14, Around, (7, 14), '\'', 1),
                 ],
             ),
             (